During nuclear fission, a fissionable nuclear fuel is bombarded with neutrons, converting the fuel into two or more fission products of lower atomic number. In addition to the fission products, this reaction produces a net increase in neutrons which, in turn, bombard additional nuclear fuel, thus forming a controlled chain reaction. During steady-state operation, neutron absorbers are used to ensure that there is no net increase in the number of neutrons available to produce fission reactions. One group of such neutron absorbers is known as burnable absorbers.
Burnable absorbers, also known as "burnable poisons," are so called because their ability to absorb neutrons decreases with increased exposure to nuclear fission. This characteristic can be beneficially used. For example, all nuclear fuels became depleted after a period of operation and must eventually be replaced. Since fuel replacement is costly, it is desirable to increase the lifetime of the fuel by initially providing as much fuel as possible. Fuel, however, is at its highest reactivity when first introduced into the reactor and, at this stage, it is imperative that excess neutrons be absorbed to maintain the reactor core in a subcritical condition.
Such excess neutrons are absorbed by introducing a burnable absorber in the core. The burnable absorber is one or more specific isotopes within an element. For example, Er.sup.167 is a burnable absorber which occurs in nature within the element erbium. In absorbing neutrons, the burnable absorber is converted to an isotope having a lower neutron capture cross-section, thus reducing the overall ability of the burnable absorber to absorb neutrons. At the same time, the fission process produces more and more fission product absorbers (poisons) which create the ability of the fuel to absorb neutrons without causing fission. For a well-designed nuclear reactor, the buildup of fission product poisons is about offset by the depletion of burnable absorbers.
Burnable absorbers can be mixed with the fuel, coated on the outer layer of the fuel, provided in the core of the fuel, or provided separate from the fuel. Materials for burnable absorbers include boron, gadolinium, samarium, europium, cadmium, hafnium, dyspropsium, indium, erbium and other materials which, upon absorption of neutrons, display a net overall decrease in neutron capture cross section.
The elements used for burnable absorber materials naturally occur in isotopic mixtures. As illustrated in the following tables, each isotope has a different thermal neutron capture cross-section:
______________________________________ Properties of Naturally Occurring Erbium Isotopes Thermal Neutron Capture Cross-Section Isotope Mass Natural Abundance (Barns) ______________________________________ 162 .136% 160 164 1.56% 13 166 33.41% 10 167 22.94% 700 168 27.07% 1.9 170 14.88% 6 ______________________________________
______________________________________ Properties of Naturally Occurring Gadolinium Isotopes Thermal Neutron Capture Cross-Section Isotope Mass Natural Abundance (Barns) ______________________________________ 152 .2% 125 154 2.15% 102 155 14.73% 61000 156 20.47% 1000 157 15.68% 254000 158 24.87% 3.5 160 21.90% 0.77 ______________________________________
U.S. Pat. No. 4,711,768 describes a scheme wherein liquid chromatography is used to separate those gadolinium isotopes having the highest thermal reutron capture cross-section (i.e., 155, 156, and 157) from the other isotopes for use in burnable absorbers. The amount of gadolinium that can be added to the nuclear fuel is apparently limited because fuel thermal conductivity and melting temperature decrease as gadolinium concentration increases. Thus, the cost-to-benefit ratios of gadolinium can be decreased by isolating the isotopes having a high capture cross-section from those having low capture cross-section.
Fractionation of naturally occurring isotopes into individual components, however, can be quite expensive, thus reducing the cost-saving benefits of the separation. Furthermore, contrary to previous teachings, it was found that adequate cost savings can be realized by removing only a single isotope. For example removal of the 166 isotope of erbium either completely or to a level of about 50% or more of the original 166 isotope concentration will obtain substantial benefits.
As another example, in the case of gadolinium, significant cost savings can be realized by the removal of the 156 isotope. This is contrary to the above teaching which suggests that only those naturally occurring isotopes having low thermal neutron capture cross-sections should be removed. It was also found that although the inclusion of isotopes with low capture cross-sections may tend to decrease thermal conductivity and melting temperature, changes in thermal neutron capture cross-section with operation should be of higher concern when providing cost-effective burnable absorbers. Again, taking gadolinium as an example, it should be noted that the 156 isotope has a thermal neutron capture cross-section of about 1000. After capturing a neutron, however, this isotope converts to the 157 isotope, which has a capture cross-section of 254,000. Since burnable absorbers are designed to decrease in overall thermal neutron capture cross-section over time, the 156 isotope is undesirable and should be removed. The same is true for the erbium 166 isotope which, upon neutron absorption to form the 167 isotope, undergoes a 70-fold increase in thermal neutron capture cross-section.
Specific methods are available for the removal of selected isotopes from an isotopic mixture to provide a depleted isotopic mixture. One such method is atomic vapor laser isotope separation ("AVLIS"). AVLIS was developed for large scale uranium enrichment applications at the Lawrence Livermore National Laboratory. AVLIS functions by first heating and vaporizing a sample of interest, followed by laser irradiation at a wavelength specifically selected to ionize only the isotope to be depleted. Once ionized, the isotope is removed using electric fields, leaving behind the depleted isotopic mixture of interest.